1. Field of the Invention
The present invention relates generally to a servo positioning system in a direct access storage device (DASD) and more particularly to a servo demodulator method and apparatus for use with transducer heads having instabilities.
2. Description of the Prior Art
Computers often include auxiliary memory storage units having media on which data can be written and from which data can be read for later use. Disk drive units incorporating stacked, commonly rotated rigid magnetic disks are used for storage of data in magnetic form on the disk surfaces. Data is recorded in concentric, radially spaced data information tracks arrayed on the surfaces of the disks. Transducer heads driven in a path toward and away from the drive axis write data to the disks and read data from the disks.
All DASD units must have a method to position each data head over the proper radial location to write a track and again, to position it very close to the same location to read the track. With the higher level files using a voice coil type of actuator, a feedback mechanism must be provided to locate and stably hold the head on a given track. Typically, track accessing and track following are provided utilizing a magnetically written pattern in the DASD unit. A dedicated servo system employs one surface of one of the disks in the DASD on which to have all the tracking and access information. A sector servo system uses small portions of tracks between each or between several sectors on each track of each data surface to provide the tracking and access information. A hybrid servo system uses both to obtain advantages of each type of servo.
Phase demodulators are known for servo control in a DASD. In some known phase demodulators, the phase information stored in the servo patterns is derived by extracting a fundamental harmonic, such as 2.5 Mhz, from the readback signal by a bandpass filter with a third harmonic trap. The first harmonic signal is then amplified and clipped, resulting in a square wave with precise phase transitions based on the zero crossings of the first harmonic signal. This square wave is then exclusive ORed or XORed with a 2.5 Mhz crystal oscillator and the transition differences between these two signals are integrated over a certain period. The integrator value at the end of this period is proportional to the head offset relative to the track center line. This value is referred to as the position error signal (PES). The problem with this arrangement is that the generation of the first harmonic signal is based upon amplitude rather than phase.
A problem exists to generate a position error signal (PES) from a phase modulated (PM) servo pattern in the presence of magnetic recording head instability. When instabilities are present in the readback signal, a PES generated by amplitude modulation (AM) is inferior to phase modulation (PM). Phase modulation (PM) is well known to have better noise immunity at an adequate signal-to-noise (S/N) than amplitude modulation. Instability of a transducer head affects the accuracy of the position error signal (PES) when the reference tracks are being read. Offset errors of up to 100 microinches have been seen between reference track updates. These offset errors usually occur if an unstable MR head is used for writing between reference track updates. Such instabilities are common with current magnetic head designs for DASD including mini-monolithic, thin film inductive, and magneto-resistive (MR) transducer heads. As head geometries continue to be reduced, magnetic instability problems will likely worsen.